Typically, one or more optical fiber leads for the transmission of optical signals or lightwaves are disposed within a protective casing to define an optical fiber transmission cable. Each optical fiber serves as a light waveguide and typically consists of a fiber that is coated to protect and preserve the strength of the optical fiber, to prevent damage during handling and to prevent environmental conditions from attacking the glass fiber. In addition, coatings are applied to decrease the microbending of optical fibers, which can reduce their efficiency in transmitting optical signals.
A typical optical fiber is shown in FIG. 1. The optical fiber 1, includes a central, fiber optic glass core 2, through which optical signals or light waves are transmitted. The glass material used to manufacture the core has a high refractive index, which is conductive to optical signal transmission.
Surrounding the core 2 is a layer of glass cladding 4, which is made from a glass material having a low refractive index. The glass cladding 4 is designed to refract the optical signals being transmitted through the central glass core 2 back into the core to allow them to be efficiently transmitted through the length of the fiber optic cable.
The glass cladding 4 is surrounded by a primary coating 6, which is preferably made of a highly elongated silicone, urethane or like material, which has a low modulus. The purpose of the primary coating 6 is to distribute any stresses applied to the exterior of the optical fiber around the circumference of the fiber. Since external stresses that are applied to an optical fiber and in particular, stresses that are applied to a single point on a fiber seriously effect the transmission of optical signals therethrough, the primary coating 6 acts as a "cushion" of sorts to minimize optical signal losses.
However, materials that provide the required stress distribution and cushioning characteristics for primary coatings do not offer adequate handling and abrasion resistance characteristics. In fact, typical primary coating materials would easily release from the optical fiber cladding if they were merely handled by an installer or maintenance technician. Therefore, in order to maintain the primary coating 6 in place around the raw optical fiber, a secondary coating 8 is applied on top of the primary coating. The secondary coating 8 is made out of a material that provides adequate handling and abrasion resistance characteristics. Typically, secondary coatings are made out of polyepoxy acrylates or like materials.
Additionally, since fiber optic cables typically include more than one optical fiber, many optical fibers are color-coded to aid in installation and splicing operations. Thus, secondary coating materials are manufactured in a variety of colors.
One or more coated fibers are then typically surrounded by a protective sheath to form a buffer. Optical fibers may be either in a tube but "loose" or "tight" buffered. In a loose buffer tube, an example of which is shown in FIG. 2 and is generally designated as 10, there is a substantial volume 12 intermediate an optical fiber 1 and the buffer tube sheath 14. This volume is typically filled with a gel-type buffer tube filling material 16, which allows the optical fiber(s) 1 to "float" within the buffer tube 10. This greatly reduces the stresses applied to the optical fibers, themselves, as the buffer tube is handled during manufacturing, installation, operation and maintenance processes.
On the other hand, in a tight-buffered optical fiber 20 (FIG. 2A), substantially the entire volume within the buffer tube 24, is occupied by an optical fiber 1 and no gel-type filling compound is employed. The buffer tube is designed to provide environmental protection for the optical fiber and to provide the necessary spacing between fibers to allow fiber optic cables to be installed and spliced using industry-standard connectors.
Since the materials used to date to form buffer tubes are typically opaque thermoplastic materials, in order to conform to the color-coding requirements of the individual optical fibers, prior art buffer tube materials are, likewise, color-coded. As can be appreciated, since optical fibers are manufactured in up to 24 different colors, manufacturers of tight-buffered fiber and manufacturers of fiber optic cables must manufacture and stock a like number of buffer tube thermoplastics. This, of course, creates manufacturing and stocking concerns.
A typical, prior art manufacturing line useful for producing thermoplastic tight buffer coatings to optical fibers is shown in FIG. 3. The manufacturing line 50 begins with a supply of optical fiber 1, which is provided to the line from an optical fiber payoff 52. A guide 53 ensures that the optical fiber 1 is oriented properly before it is pulled through an extrusion system 54. The extrusion system 54 is used to extrude a standard, prior art thermoplastic material around the optical fiber to form the tight-buffer. The extrusion system 54 includes an extruder 55, which melts the raw thermoplastic material into a molten state and extrudes the molten thermoplastic onto the optical fiber through a die (not shown). Since the extrusion process uses heat to melt the thermoplastic tight buffer material, following the extruder 55 is a cooling trough 56, through which the tight-buffered optical fiber passes in order to cool the buffer material back into its solid state. Once the, now tight-buffered optical fiber exits the cooling trough, it is pulled through a second, exit guide 53' by a capstan 58. The tight-buffered optical fiber is then taken up on a take-up spool 59.
Since prior art buffer materials are typically thermoplastics, and are applied using manufacturing processes similar to the one described above, there are significant startup, shutdown and maintenance issues associated with buffer tube production lines. For example, during startup, until the die used to extrude the thermoplastic buffer material is maintained at its preferred temperature for some period of time, there will be flaws in the buffer tube extrusion. Thus, an initial length of the tight-buffered optical fiber will need to be discarded. There is also the requirement that an extruded buffer must be precisely concentric with the fiber itself in order to eliminate the possibility of applying unwanted stresses to the fiber.
Additionally, if tight-buffered optical fibers are required of a different color, then the production line must be shut down and the die must be cooled and thoroughly cleaned or replaced before the line can be restarted using a different color thermoplastic tight buffer material. As can be appreciated, in addition to the waste that will be generated during the startup process, this will also cause significant delays in the production process.
Another limitation caused by prior art, thermoplastic tight buffer materials is manufacturing line speed. Typically, thermoplastic tight buffer application is limited to about 175 meters per minute. This limitation results from the very nature of thermoplastic materials. Thermoplastics are solids at ambient temperatures. In order to be applied around an optical fiber they need to be heated to between 475.degree. F. and 575.degree. F. in order to melt them into their liquid state. Once melted, the liquid thermoplastic materials must then be extruded around the optical fiber using an extrusion die. This process introduces shrinkage control and concentricity of extrusion issues into the equation.
Since optical fibers are generally very small in diameter, for example 250.mu., small diameter dies are required. Small dies result in high sheer stresses being applied to the molten thermoplastic as it is forced therethrough. This can result in the loss of laminar flow through the die, which in turn can result in non-uniformities and inconsistencies in the resulting tight buffer. Of course, non-uniformities and inconsistencies provide a greater likelihood of optical signal transmission loss and fiber failure.
Finally, after the thermoplastic buffer tube material is applied to a fiber, it must be cooled to return the material to its solid state. This is typically accomplished in a cooling trough, where the material is gradually reduced in temperature to minimize the likelihood of flaws created by the rapid contraction of the materials as they are cooled. Thus, cooling troughs can be lengthy, which requires a great deal of space associated with tight bufferproduction lines.
Accordingly, there is a need for a tight buffermaterial that can be applied at high rates of speed, at ambient temperatures and rapidly through-cured in place on an optical fiber. Additionally, it would be especially advantageous if the tight buffermaterial were substantially transparent to allow the optical fiber color to be seen therethrough, which would eliminate the necessity to stock a plurality of different color tight buffermaterials. Finally, a "foaming" process can be used to apply the tight buffermaterial to the optical fiber, which would reduce the amount of tight buffermaterials utilized and reduce the weight and cost of the materials.